July 29, 2011

Initial settlement of the Americas: recurrent gene flow with Asia

AJPA DOI: 10.1002/ajpa.21564

Evaluating microevolutionary models for the early settlement of the New World: The importance of recurrent gene flow with Asia

Soledad de Azevedo et al.

Abstract

Different scenarios attempting to describe the initial phases of the human dispersal from Asia into the New World have been proposed during the last two decades. However, some aspects concerning the population affinities among early and modern Asians and Native Americans remain controversial. Specifically, contradictory views based mainly on partial evidence such as skull morphology or molecular genetics have led to hypotheses such as the “Two Waves/Components” and “Single Wave” or “Out of Beringia” model, respectively. Alternatively, an integrative scenario considering both morphological and molecular variation has been proposed and named as the “Recurrent Gene Flow” hypothesis. This scenario considers a single origin for all the Native Americans, and local, within-continent evolution plus the persistence of contact among Circum-Arctic groups. Here we analyze 2D geometric morphometric data to evaluate the associations between observed craniometric distance matrix and different geographic design matrices reflecting distinct scenarios for the peopling of the New World using basic and partial Mantel tests. Additionally, we calculated the rate of morphological differentiation between Early and Late American samples under the different settlement scenarios and compared our findings to the predicted morphological differentiation under neutral conditions. Also, we incorporated in our analyses some variants of the classical Single Wave and Two Waves models as well as the Recurrent Gene Flow model. Our results suggest a better explanatory performance of the Recurrent Gene Flow model, and provide additional insights concerning affinities among Asian and Native American Circum-Arctic groups.

As with most papers dealing with the "peopling of the Americas," this one contains gems of data that contradict the current consensus but which the authors try to paper over with some consensus-friendly reassurance statements.

E.g., Fig. 2 on p. 8 (Minimum Spanning Tree) clearly shows that Northeast Asians (at least) are a subset of the Amerindian variation, with only Eskimos on the American side forming a subset of Northeast Asian variation. Northeast Asians are as different from EOW (Zhoukoudian) as it gets, hence they can't be derived from it. But they can be derived from Amerindians who in turn, despite of their low population size, possess enough diversity to span the two extremes of Old World morphological variation. The simplest way to interpret this is that the "mongoloid" complex originated in the Americas at the end of the Ice Age and as the new morphological type migrated out, some people stayed behind to become Eskimos and some other Northern Amerindians (Na-Dene). This is perfectly consistent with the levels of linguistic diversity in North America (high) vs. Northern Asia (low-moderate) as well as with archaeology: the famed fluted projectile points are associated with younger age in the North (Mesa culture in Alaska, Uptar in NE Asia) than in the south of North America. In addition, the earliest attestation of "Mongoloid" morphology are again in North America, and not in Asia. (This could be an artifact of sampling but in the light of all the data it may not be an artifact after all.)

What this means for genetics is that at the end of the Ice Age there was an admixture event that brought Amerindian genes into the Old World, thus skewing diversity levels between the Old World and the New World.

I'm fairly certain that modest gene-flow between the Americas and Asia has occured continuously since the first permenant settlement from the Beringian coast.

I think whatever sea crossings happened between Asia and the Americas after the submergence of Beringia were not continuous but there were certain relatively short periods of sea crossings, especially within the last few thousand years during the pre-Columbian Age as a result of some improvements in navigation technologies.

"I think whatever sea crossings happened between Asia and the Americas after the submergence of Beringia were not continuous but there were certain relatively short periods of sea crossings, especially within the last few thousand years during the pre-Columbian Age as a result of some improvements in navigation technologies."

Well, the evidence we have shows 3 conclusive incidences of whole ethnic groups crossing the Bering sea in sufficient force to leave linguistic proof. I would suggest smaller migrations were probably quite frequent during the same period, enough to constitute appreciable gene flow.

The Na-Dene exceptionality suggests another older migration, so with 3 certain and 1 strongly probable migration, we might even be looking at quite a few individuals entering and leaving North America and adding DNA every generation, in smaller migrations.

The Na-Dene exceptionality suggests another older migration, so with 3 certain and 1 strongly probable migration, we might even be looking at quite a few individuals entering and leaving North America and adding DNA every generation, in smaller migrations.

By "certain relatively short periods of sea crossings" I was referring to the Na-Dene and Eskimo-Aleut migrations in particular. I don't exclude the possibility of other pre-Columbian sea crossings from Asia to the Americas or vice versa. BTW, the dating of the Na-Dene migration is very much open to debate; I favor recent dates.

"the dating of the Na-Dene migration is very much open to debate; I favor recent dates".

I think you are probably correct there.

"Genetic analyses done so far have shown that population bottleneck(s) happened during the human dispersal to the Americas from Asia".

Autosomal analysis? Perhaps in the Y-chromosome line, but in the mtDNA line? At least four separate haplogroups from the two separate non-African clades. That suggests that a fairly varied population made it into America, and survived there.

Autosomal analysis? Perhaps in the Y-chromosome line, but in the mtDNA line? At least four separate haplogroups from the two separate non-African clades. That suggests that a fairly varied population made it into America, and survived there.

Terry, I and the current consensus among geneticists are not maximalist when it comes to the bottleneck(s) during the pre-Columbian human dispersals from Asia to the Americas, as probably the bottleneck(s) was/were partial and multiple migrations happened to the Americas. Even you will acknowledge the restricted genetic structure of the New World Mongoloids compared to the Old World Mongoloids.

"Even you will acknowledge the restricted genetic structure of the New World Mongoloids compared to the Old World Mongoloids".

Perhaps. But what struck me when I visited the USA was how different Native Americans from different regions were. I was surprised as I'd expected them all to look much the same. And that was just within the great plains region of the USA, from Denver to mephis and from San Antonio to Kansas, although I also met Native Americans from outside that region. Interestingly I also met some Hawaiians, who I at first assumed were Maori from New Zealand. Not surprising I thought that of course.

Perhaps. But what struck me when I visited the USA was how different Native Americans from different regions were. I was surprised as I'd expected them all to look much the same. And that was just within the great plains region of the USA, from Denver to mephis and from San Antonio to Kansas, although I also met Native Americans from outside that region. Interestingly I also met some Hawaiians, who I at first assumed were Maori from New Zealand. Not surprising I thought that of course.

Is there a detailed craniometric analysis of Native Americans available on the internet?

BTW, being genetically restricted does not mean being genetically homogeneous. Native American groups (even excluding Eskimos, and if you wish, also Na-Dene speakers) might still have significant genetic differences between each other (mtDNA shows some signs of that).

And African. To me most Cherokee had an African element which the Cherokee assured me was pre-European. But the Cherokee had African slaves before they were forced west so I presume that is the time of mixture.

And African. To me most Cherokee had an African element which the Cherokee assured me was pre-European. But the Cherokee had African slaves before they were forced west so I presume that is the time of mixture.

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